The testes of male animals including humans, is divided into two function compartments. One compartment which is responsible for the production of sperm, contains a tightly packed series of tubular coils known as the seminiferous tubules. The seminiferous tubules make up the majority of the testes structure. Hormone producing cells located within the seminiferous tubules which assist in the transformation of germ cells to sperm cells are known as Sertoli cells. The other compartment of the testes contains Leydig cells. Leydig cells are responsible for the production of androgens or male sex hormones. These Leydig cells are interspersed between the various coils of the seminiferous tubules.
For years it was known that when a certain chemical bound to specific membrane receptors on Leydig cells, a series of reactions is initiated which result in the production and secretion of androgens. "Androgen" refers to a family of male sex hormones which include, without limitation, dihydrotestosterone, androstenedione and testosterone. Androgens such as testosterone, play a key role in the development of male sexual characteristics such as growth of the penis, muscles and beard as well as deepening of the voice. Testosterone secreted by the Leydig cells was also known to bind to specific receptors in the Sertoli cells and this binding was known to be important in sperm production.
The compound that was known to bind to Leydig cells was Lutenizing Hormone (LH) which is secreted by the pituitary gland under the brain. (A functionally analogous compound produced in females, Chorionic Gonadotropin (CG) or human Chronic Gonadatropin (hCG) can also bind to LH receptors on Leydig cells. CG is easier to obtain than LH and, therefore, clinical studies often use CG instead of LH. Because they are functionally interchangeable, they are often referred to as "LH/CG".) It was originally thought that the binding of LH/CG to the Leydig cell was the sole mechanism for initiating the production of androgens. Another compound, Follicle Stimulating Hormone (FSH) was known to be released by the pituitary gland under the brain and was known to bind to specific membrane receptors on the Sertoli cells. The binding of FSH along with testosterone, to the Sertoli cells were thought to be key to sperm production.
These simplistic explanations of the operation of the Sertoli and Leydig cells were fine for a basic understanding of testicular biology. However, there were certain situations which these explanations just could not accommodate. For example, it was found that when Sertoli cells were damaged, the Leydig cells surrounding them underwent rapid enlargement, without a change in LH levels. This suggests that the life cycle of both types of cells are interdependent, at least to some extent. Furthermore, there is a disease in children that causes the Leydig cells to produce large amounts of testosterone, even in the absence of LH. It has long been believed that some other substance, contained within the testes, activates the Leydig cells to produce androgen in this instance. Finally, in the fetus, testosterone or other androgens are produced before the Leydig cells LH receptors are operational. These observations suggested a symbiotic relationship between the Sertoli cells and the Leydig cells above and beyond their respective known roles in sperm cell production.
The understanding of the chemical interaction between the Sertoli cells and the Leydig cells has gradually grown, particularly over the last ten years. During that period, the results of a number of studies were released and those studies appear to support the symbiotic relationship of Sertoli cells and Leydig cells. For example, when porcine Leydig cells were cultured alone in chemically defined medium, the cells exhibited regression and showed the loss of smooth endoplasmic reticulum and swelling of the mitochondria. These changes were not prevented by the addition to the medium of either LH which stimulates Leydig cells or FSH which stimulates Sertoli cells.
In contrast, the co-culturing of Leydig and Sertoli cells resulted in Leydig cells that retain the smooth endoplasmic reticulum at the same level as exhibited in Leydig cells from intact animals. Furthermore, the addition of FSH to the co-culture appeared to stimulate Leydig cell development. (Tabone, et al., '84). Such observations have led numerous investigators to search in various unfractionated biological fluids from the testes for factors which affect Leydig cell function. These studies have led to the identification of both inhibitory and stimulatory activities.
Stimulatory activities include a group of biological activities that appear to be stable to heat, acids, or organic solvents and are capable of increasing both basal and LH-stimulated testosterone secretion. These factors have an average molecular weight of between about 5,000 and about 10,000. (Syed Int. J. Androls' '86) (Sharp & Cooper '84).
Other investigators have identified another group of Leydig cells stimulatory activities that are heat and enzyme sensitive and appear to be proteins with average molecular weights between about 10,000 and 60,000. These factors can also stimulate both basal and LH-stimulated testosterone secretion. (Sharpe INSERM '84); (Sharp & Bartlett '85); (Sharp Mol. Cell Endo. '88); (Benahmed Am. J. Phys. '85); (Benahmed J. Ster. Biochem '86); (Parvinen Mol. Cell Endo. '84); Papadopoulus '87a); (Papadopoulus '87b); (Verhoeven & Cailleau '85); (Verhoeven & Cailleau '87); (Janecki Mol. Cell Endo. '85); (Jansz Biol. Reprod. '87); and (Liu & Dahl '88).
The present inventors have isolated a hitherto unknown protein which has important and unique functionalities and which has potentially significant therapeutic properties. This factor, known as Leydig Cell Stimulator, or "LCS", acts in ways which are not characteristic of other identified stimulatory factors. Furthermore, LCS acts to enhance and in some instances synergistically enhance the production of androgens such as testosterone when co-administered with LH/CG. This discovery not only provides a better overall understanding of the interplay between Sertoli cells and Leydig cells, but may also be useful in the treatment of infertility, low sperm count, premature or late onset of puberty as well as other conditions.